Experimental study of flame characteristics and stability regimes of biogas – Air cross flow non-premixed flames

Abstract

Biogas is an alternative fuel that typically contains around 45% carbon-dioxide by volume, besides methane. Due to the inherent content of carbon-dioxide, it is necessary to study the flame characteristics and stability limits in cross-flow non-premixed burners. In this study, cross-flow non-premixed flames, where biogas is injected through a horizontal porous plate and air is blown parallel to the fuel injector, are studied systematically. In order to increase the stable operating regime, devices such as backward facing steps and cylindrical bluff-bodies are commonly employed. Different step-heights and locations from leading edge of the fuel injector are considered for the cases with backward facing steps. A rectangular cylindrical bluff-body is also used as a flame stabilizing obstacle. Baseline cases are studied without any backward facing step or cylindrical bluff-body. Volume flow rate of biogas is varied from 36 liter per hour to 360 liter per hour. Air velocity is varied in the range of 0.2 m/s to 3.0 m/s. For a given fuel velocity, air velocity is gradually increased in order to record the transition of flame from one regime to another. Flame stabilization is carefully assessed by monitoring the high definition direct flame photographs captured from front and top views, for all the cases. The cases are repeated at least three times to ensure repeatability. Stability maps are plotted as a function of fuel velocity and air velocity for all the cases. For cases with backward facing steps, both step height and its location play an important role in delineating the boundaries of the flame regimes. Parametric variations show interesting features. Bluff-body flames become quite oscillatory and three dimensional at higher air velocities. For this case, stability maps of flames from biogas and pure methane are compared.